Unfortunately no way to change it. What we did was change the 'right arrow button' on a yealink phone to a speed dial for '777'. We edited this on the template and sent it out to all the phones, so the user need only press one button and it will dial 777.

Yeah, not my issue unfortunately. I have no intention of using the feature, but 777 is currently my main number for reception, meaning I'm going to have to reeducate all our users when we migrate to 3cx.